1. Field of the Invention
The present invention relates to improvements for thermoplastic films, particularly thermoplastic films used in the manufacture of bags including trash bags.
2. Description of the Related Art
Thermoplastic films are used in a variety of applications. For example, thermoplastic films are used in sheet form for applications such as drop cloths, vapor barriers, and protective covers. Thermoplastic films can also be converted into plastic bags, which may be used in a myriad of applications. The present invention is particularly useful to trash bags constructed from thermoplastic film, but the concepts and ideas described herein may be applied to other types of thermoplastic films and bags as well.
Depending on the application, the use of thermoplastic film presents technical challenges due to the fact that thermoplastic film is inherently soft and flexible. Specifically, thermoplastic films are susceptible to puncture and tear propagation. In some instances, it may be possible to increase the thickness of the film or select alternative polymers to enhance the physical properties of the film. However, these measures may increase both the weight and cost of the thermoplastic film. Moreover, such modifications of thermoplastic film may not be practical in every situation. In light of the technical challenges associated with thermoplastic film, techniques and solutions have been developed to address the need for improved shock absorption to reduce the likelihood of puncture. For example, it is known to impart stretched areas into thermoplastic films as a means of inducing shock absorption properties into the film.
U.S. Pat. No. 5,205,650, issued to Rasmussen and entitled Tubular Bag with Shock Absorber Band Tube for Making Such Bag, and Method for its Production, discloses using thermoplastic film material with stretchable zones wherein the film material has been stretched in a particular direction with adjacent unstretched zones that extend in substantially the same direction. The combination of the stretched zones and adjacent unstretched zones provides a shock absorber band intended to absorb energy when the bag is dropped. Specifically, when a bag is dropped or moved, the contents inside the bag exert additional outward forces that might otherwise puncture or penetrate the thermoplastic film. However, the shock absorber bands absorb some energy and may prevent puncture of the thermoplastic film.
Additional examples of a thermoplastic film material designed to stretch are disclosed in U.S. Pat. No. 5,518,801, issued to Chappell and entitled Web Materials Exhibiting Elastic-Like Behavior (hereafter Chappell '801), and U.S. Pat. No. 5,650,214, issued to Anderson and entitled Web Materials Exhibiting Elastic-Like Behavior and Soft Cloth-Like Texture (hereafter Anderson '214). In Chappell '801 and Anderson '214, as well as several closely related references, an arrangement of raised ribs is embossed into the thermoplastic film to provide a stretchable film. Anderson '214 further discloses using a plurality of embossed ribs defining diamond-shaped areas with a network of narrow, unembossed regions extending between the large number of diamond-shaped areas. In the disclosed embodiment of Anderson '214, the unembossed areas form a network of linear unembossed regions extending in two perpendicular directions. The stretch properties of these films created by the use of embossed and unembossed regions is frequently referred to as induced elasticity of the film, which facilitates elastic response in a particular direction across the area of the embossed regions and the narrow, unembossed regions separating the embossed regions.
While the foregoing references specifically address the desire to generally increase the shock absorption of the thermoplastic film to enhance the stretch properties, or induced elasticity, of the film, it is further known to provide regions of induced elasticity where the induced elasticity is oriented in different directions in different portions of the film or, more specifically, in a thermoplastic bag made from such film. For example, U.S. Pat. No. 6,394,651, issued to Jackson et al. and entitled Flexible Bags Having Enhanced Capacity and Enhanced Stability (hereafter Jackson '651), discloses a thermoplastic bag having an upper portion that has induced elasticity in a fill direction. i.e. along the length of the bag, while a lower portion of the bag has induced elasticity in a perpendicular direct, i.e. across the width of the bag. In the embodiment disclosed in Jackson '651, the boundaries between the areas of induced elasticity are straight and linear. As a result, a disproportionate amount of stress can be exerted along the straight boundaries, which can result in tears propagating along the boundaries. Additionally, substantially the entire surface area of the thermoplastic bag disclosed in Jackson '651 has induced elasticity. This negatively impacts the dart impact properties of the thermoplastic bag.
U.S. Pat. No. 6,513,975, issued to Jackson et al. and entitled Bag with Extensible Handles (hereafter Jackson '975), discloses additional embodiments of thermoplastic bags with multiple areas of induced elasticity in addition to the embodiment disclosed in Jackson '651. One additional embodiment disclosed in Jackson '975 has a lower portion with induced elasticity across the width of the bag, an upper, centrally-located area of induced elasticity in the fill direction, and additional areas of induced elasticity along the sides of the bag near the top with induced elasticity in an angular direction. Additionally, another embodiment disclosed in Jackson '975 discloses a thermoplastic bag with areas of induced elasticity extending along substantially the entire length of the bag, in the fill direction, separated by areas lacking induced elasticity. Like the embodiment of Jackson '651, these embodiments also feature boundaries for each of the areas of the induced elasticity that are straight and linear and, thus, can result in tears propagating along the boundaries. More importantly, in every depicted embodiment of Jackson '975, the area of induced elasticity exceeds 50% of the surface area of the film, which significantly reduces the dart impact properties of the film.
Ultimately, when applied to a thermoplastic bag, the foregoing teachings can result in additional capacity for a thermoplastic bag compared to a thermoplastic bag of the same dimensions without induced elasticity. However, the stretchable thermoplastic films described above are accompanied by certain disadvantages as well. In particular, the embossing of ribs across the surface area of the thermoplastic film necessarily thins the film and typically has a detrimental effect on the dart impact properties of the film. Additionally, disproportionate amounts of stress are exerted along straight boundaries between areas of induced elasticity and areas lacking the same resulting in a weakening of the film along straight boundaries. The thinner film has a greater likelihood of failing as a result of an item piercing the film. In short, there is an increased likelihood of punctures being introduced into the film and, in some cases, of the propagation of tears across the surface of the thermoplastic film as well.
It would be advantageous to provide a thermoplastic bag that offers the advantage of increased capacity from induced elasticity without substantively sacrificing dart impact resistance across the surface of the thermoplastic film as a whole. The present invention addresses these needs.